Thousands of Egyptians celebrate on in Cairo's Tahrir Square following the announcement that Muslim Brotherhood member Mohamed Morsi would be Egypt's next president.

Khaled DesoukiAFP/Getty Images

Muslim Brotherhood candidate Mohammed Morsi is Egypt's new president, the country's electoral commission announced on Sunday. A massive crowd in Cairo's Tahrir Square erupted in cheers at the announcement.

Morsi's election is a victory for Islamist groups as well as those who saw his candidacy as a way to clear out last remnants of ousted leader Hosni Mubarak's regime.

The AP has the numbers, saying, "Morsi won with 51.7 percent of the vote versus 48.3 for [former Prime Minister Ahmed] Shafiq. Turnout was 51 percent."

Update at 6:37 p.m.: Obama Speaks To Morsi

The White House announced that President Obama spoke to Egypt's new president-elect.

Here's the statement from the White House:

"President Obama called Dr. Mohamed Morsi today to congratulate him on his victory in Egypt's presidential election. The President underscored that the United States will continue to support Egypt's transition to democracy and stand by the Egyptian people as they fulfill the promise of their revolution. He emphasized his interest in working together with President-elect Morsi, on the basis of mutual respect, to advance the many shared interests between Egypt and the United States. President-elect Morsi expressed appreciation for the call and welcomed U.S. support for Egypt's transition. The two leaders affirmed their commitment to advancing the U.S.-Egypt partnership and agreed to stay in close touch in the weeks and months ahead."

Update at 5:25 p.m.: Morsi Addresses Nation

In a televised speech, the president-elect said he carries "a message of peace" to the word, and pledged to maintain the country's international accords, a possible reference to Israel.

"I wouldn't have been here between your hands as the first elected president without ... the blood, the tears, and sacrifices of the martyrs," he said, referring to the often-deadly protests that led to the end of Hosni Mubarak regime and paved the way for Morsi's election on Sunday.

He also tried to reassure the country's minorities, by urging all Egyptians to unite. The country's minority Christians had mostly backed Morsi's rival, Ahmed Shafiq.

The Associated Press notes that Morsi did not refer to the "last-minute power grab by the ruling military."

Update at 4:07 p.m.: White House, Israel React To Morsi's Win

In a statement, the White House congratulated the Muslim Brotherhood candidate on his victory.

"We believe that it is important for President-elect Morsi to take steps at this historic time to advance national unity by reaching out to all parties and constituencies in consultations about the formation of a new government," the statement said.

It called on the new government to uphold universal rights of all Egyptians, including women and religious minorities.

In Israel, which has peace treaty with Egypt, reaction was more muted.

In a statement, Prime Minister Benjamin Netanyahu's Office said it respected the results, adding "it looks forward to continuing cooperation with the Egyptian government on the basis of the peace treaty" between the two countries.

Our original post continues:

All weekend, crowds swelled in Tahrir Square, the birthplace of Egypt's pro-democracy movement. On Sunday, supporters of the Muslim Brotherhood, ultraconservative Islamists and revolutionary youth groups mixed in the midday heat as they waited for the announcement.

Speaking to supporters of Morsi's opponent, Ahmed Shafiq, Nelsons says they don't trust the Muslim Brotherhood. She says some hope the military will conduct a coup in short order for fears of an Islamist agenda becoming Egypt's policy.

The announcement of the election results was to be the resolution of the tumultuous transition to democracy Egypt began when a popular uprising swept Mubarak from power more than a year ago.

Instead, backroom deals and last-minute moves by the military cast suspicion over the country's first free presidential vote, spiking tensions across the country.

Results from last weekend's runoff elections were originally to be announced on Thursday, but were postponed until today. Ahead of the announcement, rumors flew of fraud and secret bargains made to favor rival candidate and Shafiq, Mubarak's last prime minister.

Regardless of the election's outcome, however, it is Egypt's military that remains in power. In the 16 months since Mubarak was ousted, it has assumed control of all key branches of state. Just before elections last weekend, the ruling generals dissolved Egypt's popularly elected parliament, which was dominated by the Muslim Brotherhood. Shortly after polls closed, they amended Egypt's constitution to strip power from the incoming president. Powers of arrest and detention have also been broadened.

Extra security forces have been deployed around the country since Saturday, the AP reports. Armored vehicles, troops and riot police are stationed at Cairo's airport, around Parliament and throughout the streets of the city. Sunday is a work day in Egypt, but employees were sent home early over concerns of new violence – many stopping to stock up on food and jewelry along the way.